发生异常：“字符串”类型不是“模式”类型的子类型

Question

我最近将一个功能齐全的应用程序转移到 dart null safety 并引发了一些错误

此 API 调用已调整，但现在我收到错误 flutter: Exception Happened: type 'String' is not a subtype of type 'Mode'

我通过获取用户位置的函数调用 API。

重申一下，这在升级 dart 之前运行良好，所以我相信这是适应 null 安全性的问题。

API 调用（抛出异常的地方）

Future<Stations?> fetchStations() async {
    var client = http.Client();
    Stations? stations; 
     
   Position position = await Geolocator.getCurrentPosition(desiredAccuracy: LocationAccuracy.high);


    var lat = position.latitude;
    var long = position.longitude;
    
try{
    var response = await client.get(Uri.parse('https_call'));
    if (response.statusCode == 200) {
   var jsonString = response.body;
   var jsonMap = json.decode(jsonString);
   
   stations = Stations.fromJson(jsonMap);
   //print(stations);
  }
} catch(e) {

  print("Exception Happened: ${e.toString()}");
}
 
  return stations; 
}

调用API的函数

Future<void> showAnchoredMapMarkers() async {
  var stations = await fetchStations();
  for (Station stations in stations!.stations) {
    GeoCoordinates geoCoordinates = GeoCoordinates (stations.place.location.lat, stations.place.location.lng);
    var id = stations.place.id;
    
    _addCircleMapMarker(geoCoordinates, 0);
    _addPOIMapMarker(geoCoordinates, 1);
  
  }
}

更新

class Station {
    Station({
        this.place,
        required this.transports,
    });

    Place place;
    List<Transport> transports;

    factory Station.fromJson(Map<String, dynamic> json) => Station(
        place: json["place"] == null ? null : Place.fromJson(json["place"]),
        transports: json["transports"] == null ? null : List<Transport>.from(json["transports"].map((x) => Transport.fromJson(x))),
    );

    Map<String, dynamic> toJson() => {
        "place": place == null ? null : place.toJson(),
        "transports": transports == null ? null : List<dynamic>.from(transports.map((x) => x.toJson())),
    };
}

这样的行

place: json["place"] == null ? null : Place.fromJson(json["place"]),

全部用红色下划线标有错误，如

The argument type 'Place?' can't be assigned to the parameter type 'Place'.

Answer 1

发生这种情况是因为在您的 Model 文件中，您定义了 mode 的值应该是 Mode，但您将 String 传递给它。简单的解决方案是将模式的datatype 更改为String，如下所示：

class Transport {
    Transport({
        required this.mode,
        required this.name,
        required this.color,
        required this.textColor,
        required this.headsign,
    });

    String mode;
    String name;
    String color;
    TextColor textColor;
    String headsign;

   more code ...
}